The present invention relates to a preventive maintenance method and apparatus for reducing a tensile residual stress on a surface of a structural member in a reactor pressure vessel (RPV) by discharging a liquid (water) jet to the surface of the structural member, thereby preventing occurrence of stress corrosion cracking (SCC). Particularly, the present invention relates to a method and apparatus suitable for reducing the tensile residual stress of a weld portion and a weld heat-affected zone located in a narrow space or an inaccessible portion.
A water jet peening (WJP) method is known as a method that adds a compressive residual stress to a surface layer of a metal material. In the WJP method, a nozzle is set opposite to the metal material in water, and a water jet containing cavitation bubbles is discharged from the nozzle toward the metal material in the water. When the water jet collides with (or impinges on) a surface of the metal material, the cavitation bubbles are collapsed by an axial dynamic pressure. When the cavitation bubbles are collapsed, an impact pressure is produced by a water-hammering effect, and this impact pressure strikes the surface of the metal material to add the compressive residual stress.
The first prior art as to the WJP method is disclosed in Japanese Patent Laid-open No. Hei 4-362124. In this method, a WJP is performed by discharging a water jet containing cavitation bubbles from a nozzle which is directed to a metal material in water, and by impinging the water jet on a surface of the metal material while moving the nozzle along the metal material.
The second prior art applicable to an inner surface of a tube with a small diameter is disclosed in Japanese Patent Laid-open No. Hei 10-76467. In this case, a high speed liquid jet containing cavitation bubbles is discharged from a nozzle which is directed to an axial direction of a tube, and a conical baffle body which gradually reduces a cross-sectional area of a flow passage in the tube and a columnar baffle body located adjacent to the conical baffle body are coaxially provided on a downstream side from the nozzle.
In a region where the conical baffle body is provided, the cavitation bubbles are collapsed limitedly near an inner surface of the tube because a peripheral pressure of the jet gradually increase by a restriction effect of the flow passage. Fine cavitation bubbles, which are not collapsed in the above region, become nuclei of other cavitation bubbles in a local low pressure area produced by a separation phenomenon of the jet flow at a transition corner from the conical baffle body to the columnar baffle body, and generate secondary cavitation bubbles. This secondary cavitation bubbles are collapsed at a downstream side from the corner. This document also shows one example of the conical baffle body which has an apex angle of 60 in a longitudinal cross section.
The above two methods are intended to subject a metal surface to peening treatment using collapse pressures of cavitation bubbles so as to convert a tensile residual stress which initially presents in a surface layer of the metal material into a compressive residual stress.
However, the first WJP method is carried out by discharging the jet from the nozzle provided opposite to the metal material while moving the nozzle along the metal material. Accordingly, this WJP method is difficult to apply to a narrow space portion such as an outer surface of a core shroud and an inner surface of a tubular structure with a small diameter such as an in-core monitor (ICM) housing in a RPV.
The second WJP method is applicable to the ICM housing, but can not apply to the narrow space portion such as the outer surface of the core shroud. Further, in a case that the second WJP method applies to the ICM housing, the restriction effect of the flow passage becomes almost uniform in a peripheral direction in the region where the conical baffle body is provided, but an effect of generating a peeling flow by collision of the jet is not obtained.
Therefore, since the cavitation bubbles do not grow so largely, a local impact pressure (collapse pressure) applied to the inner surface of the tube is restricted (limited). The peeling flow is generated at the transition corner from the conical baffle body to the columnar baffle body, but since the corner has an obtuse angle in a longitudinal cross section, strength of the peeling flow is also restricted.
It is an object of the present invention to provide a preventive maintenance method and apparatus of a structural member in a reactor pressure vessel (RPV), capable of applying to a narrow space portion such as an outer surface of a core shroud and an inner surface of a tubular structure with a small diameter such as an in-core monitor (ICM) housing in the RPV filled with core water, and also capable of producing cavitation bubbles with high collapse pressures, and improving a residual stress on a surface of the structural member by collapsing the cavitation bubbles at a desired surface of the structural member, thereby preventing a damage such as stress corrosion cracks (SCC).
A water jet, which has collided with (or impinged on) a wall surface in a direction substantially perpendicular thereto and has influenced by the collision (or the impingement), is particularly called a collision jet. In such a collision jet, a separation flow more violent (stronger) than that caused by collision with a portion having a simple discontinuous shape is produced by an vortex flow and a turbulent flow.
As a result, when the water jet collides with the wall surface, part of cavitation bubbles contained in the water jet collapse at the wall surface. But remaining fine cavitation bubbles grow near the wall surface and new cavitation bubbles are generated near the wall surface. The present invention is based on such a feature of the collision jet.
In accordance with the present invention, a preventive maintenance method of a structural member in a reactor pressure vessel for reducing a tensile residual stress on a surface thereof, has the steps of impinging a water jet from a nozzle onto a plane surface of a deflector to thereby change direction of flow of said water jet, and impinging the water jet after being deflected onto the surface of the structural member.
In this case, since it need not to direct the nozzle to the surface of the structural member, if a spatial width of a narrow space portion (or an inner diameter of a tube with a small diameter) is larger than an outer diameter of the nozzle, this method is applicable to the narrow space portion (or the tube with the small diameter).
Further, since the water jet from the nozzle becomes a collision jet including a strong separation flow and a strong vortex flow by the impingement on (or collision with) the plane surface of the deflector, cavitation bubbles contained in the collision jet grow largely (become large). As a result of a combination of this effect and a strong water-hammering effect on the surface of the structural member, the cavitation bubbles in the collision jet give high collapse pressures to the surface of the structural member when the cavitation bubbles collapse on the surface of the structural member. That is, a high compressive residual stress can be added to the surface of the structural member. Accordingly, it can be possible to improve a residual stress on the surface of the structural member and also prevent a damage such as SCC.
Preferably, a distance between the nozzle and the plane surface of the deflector is at most 100 times (preferably at most 50 times) as large as a hole diameter of the nozzle. In this case, since the water jet collides with the plane surface of the deflector before fine cavitation bubbles contained in the water jet become large and velocity of the water jet becomes low, it is possible to reduce the amount of the cavitation bubbles collapsed by the collision with the plane surface of the deflector, and also make the fine cavitation bubbles largely grow by a collision effect with the plane surface of the deflector. Accordingly, the cavitation bubbles having high collapse pressures can be collapsed on the surface of the structural member.
Preferably also, an angle formed between a central axis passing through an opening of the nozzle and the plane surface of the deflector is in a range of 10 to 90, preferably in a range of 40 to 90, more preferably in a range of 60 to 90. In this case, since the collision jet including the strong vortex flow and the strong separation flow can be generated, fine cavitation bubbles largely grow and new cavitation bubbles are generated in the collision jet. Accordingly, the cavitation bubbles having high collapse pressures can be collapsed on the surface of the structural member.
In accordance with the present invention, a preventive maintenance method of a structural member in a reactor pressure vessel for reducing a tensile residual stress on a surface thereof, has the steps of impinging a water jet from a nozzle onto a recess of a deflector to thereby change direction of flow of said water jet, and impinging the water jet after being deflected onto the surface of the structural member.
In this case, since flow direction of a collision jet is opposed to that of the water jet from the nozzle and a water-hammering effect becomes much stronger, a strong vortex flow and a strong separation flow are generated in the collision jet. Accordingly, the cavitation bubbles having high collapse pressures can be collapsed on the surface of the structural member.
Preferably, the recess is in shape of cone with an apex angle of at least 90 (preferably at least 120) in a longitudinal cross section thereof.
Further, preferably, the structural member is a core shroud, an in-core monitor housing or a water-level measuring nozzle.
In accordance with the present invention, a preventive maintenance apparatus of a structural member in a reactor pressure vessel for reducing a tensile residual stress on a surface thereof, has a nozzle for discharging a water jet into core water in a reactor pressure vessel, a deflector having a plane surface which is impinged by said water jet to change direction of flow of said water jet discharged from the nozzle, and a support maintaining a predetermined distance between the nozzle and the plane surface of the deflector.
Preferably, the support maintains the distance between the nozzle and the plane surface of the deflector at most 100 times (preferably at most 50 times) as large as a hole diameter of the nozzle. In this case, the nozzle and the deflector supported by the support can access easily to a narrow space portion (or the inside of a tube with a small diameter). Further, a suitable distance between the nozzle and the plane surface of the deflector can be maintained certainly by the support.
Preferably also, the support maintains an angle (collision angle), formed between a central axis passing through an opening of the nozzle and the plane surface of the deflector, in a range of 10 to 90, preferably in a range of 40 to 90, more preferably in a range of 60 to 90. In this case, a suitable collision angle can be maintained certainly by the support.
Preferably also, the support has one opening for discharging the direction-changed flow of the water jet (collision jet), near the plane surface of the deflector. In this case, part of the collision jet flowing toward direction in which the opening is not provided, changes its flow direction toward the opening by making a second collision with an inner wall of the support and are discharged from the opening so as to collide with the surface of the structural member. Cavitation bubbles in the collision jet grow more largely by this second collision. Accordingly, the cavitation bubbles having high collapse pressures can be collapsed on the surface of the structural member.
Preferably also, the support has openings for discharging the direction-changed flow of the water jet near the plane surface of the deflector, the openings being arranged in a peripheral direction with respect to a central axis passing through an opening of the nozzle. In this case, surfaces corresponding to the openings can be treated with a WJP method simultaneously.
Preferably also, further has a pressurized water supply for supplying pressurized water to the nozzle.
In accordance with the present invention, a preventive maintenance apparatus of a structural member in a reactor pressure vessel for reducing a tensile residual stress on a surface thereof, has a nozzle for discharging a water jet into core water in a reactor pressure vessel, a deflector having a recess which is impinged by the water jet to change direction of flow of the water jet discharged from the nozzle, and a support maintaining a predetermined distance between the nozzle and the recess of the deflector.
Preferably, the recess is in shape of cone with an apex angle of at least 90 (preferably at least 120) in a longitudinal cross section thereof.
Further, preferably, the support has openings for discharging the direction-changed flow of the water jet near the recess of the deflector, the openings being arranged in a peripheral direction with respect to a central axis passing through an opening of the nozzle.
Further, preferably also, the recess of the deflector has spiral grooves or spiral projections for making a revolving flow of the direction-changed flow of the water jet (collision jet) with respect to the central axis passing through the opening of the nozzle. In this case, since the collision jets discharged from the openings of the support are given velocity components in the peripheral direction, the collision jets being uniform in the peripheral direction can be formed. This collision jets are suitable to treat an inner surface of a tubular structure with a WJP method.
Preferably also, further has a pressurized water supply for supplying pressurized water to the nozzle.